Injection of hydrated lime and other alkaline materials is a promising technology for control of acid, such as NOx (NO and NO2), HCl, and SOX (SO2 and SO3) from coal—and biomass-fired sources. Acid gas control is becoming obligatory due to the problems arising from increased corrosion, acid mist emissions and associated impacts to plant opacity, and the propensity of certain acid gases, such as SO3, to interfere with powdered activated carbon (“PAC”) used for mercury capture from these sources. Concerns with SO3 emissions have increased due to selective catalytic reduction reactors (“SCRs”) oxidizing sulfur dioxide to sulfur trioxide. SCRs are being installed on an increasing number of coal-fired sources for control of nitrogen oxides. SOx species are also present in flue gas at elevated levels when burning high sulfur coals. The presence of sulfur species and the ammonia reactants used for nitrogen oxide control can combine to form condensable compounds that foul or degrade air heater performance over time.
Injection of dry alkaline sorbents to control acid gas emissions continue to be used successfully at many coal-fired sources to chemically control emissions. When dry alkaline materials are injected into a gas stream for the purpose of controlling acid gases, the desired chemical reactions occur in the flue gas stream.
A major goal of any injection system is to maximize the desired reactions and minimize undesired reactions and/or interactions with the walls and mechanical systems downstream. As an example, one of the desired acid gas reactions between hydrated lime and SO3 is shown below:Ca(OH)2+SO3→CaSO4+H2Ohydrated lime+sulfur trioxide=calcium sulfate+waterOne of the major undesired reactions that occurs within the alkaline sorbent injection system is that of hydrated lime with carbon dioxide:Ca(OH)2+CO2→CaCO3+H2Ohydrated lime+carbon dioxide=calcium carbonate+waterThe rate of carbonate formation is believed to be a temperature-dependent process; that is, the higher the gas temperature the greater the rate of conversion of hydrate to carbonate. Calcium carbonate has been shown to be inversely soluble, therefore increasing temperature leads to greater carbonate deposition within the injection system. Managing the temperature of the injection system and lime carrier gas reduces the rate of formation and subsequently minimizes the deposition of calcium carbonate. Due to the presence of air in the injection system, CO2 is always available for reaction in either the carrier gas or in the flue gas that contaminates the carrier gas through leakage or recirculation into the injection system. Therefore, thermal management of the injection system is employed to successfully moderate carbonate formation, reduce lime consumption, and increase system reliability.
A carrier gas treatment system 200 according to the prior art includes an optional dehumidifier 204, a regenerative or positive displacement blower 208 (air having a pressure in the range of 3 to 25 psi), and a refrigerated air dryer 212 and/or after cooler 216. Other configurations and arrangements of the illustrated equipment are possible. This system 200 generally reduces the dew point of the conveying air to a temperature just above 32° F. Although the reaction of calcium hydroxide and carbon dioxide to air is slowed by the gas dehumidification and cooling, the degree of dehumidification and cooling is limited. Accordingly, conventional systems 200 generally experience debilitating issues with scaling, abrasion, plugging in the lines, lances, and other conveying surfaces in the system 200.
These problems increase the necessity for time-consuming and expensive manual cleaning and maintenance of the introduction system.